3. Nanomaterials for renewable energy applications.

Nanomaterials will play important roles in increasing the performance of various renewable energy generation technologies, such as photovoltaics and thermoelectric energy generations. Nanocrystals (quantum dots) of PbSe and PbS have been recently observed to exhibit multiple exciton generation (MEG), in which several excitons can be generated with a single incident high energy photon in these narrow bandgap materials. This discovery promises to allow us to go beyond the Shockley-Queisser limit for single band solar cells and greatly improve the efficiency of photovoltaic energy generation. In collaboration with Prof. John Wright’s group and Prof. Robert Hamers’ group at UW-Madison, we are investigating these charge multiplying nanocrystals (quantum dots) and nanowires of lead chalcogenides (PbSe, PbS, and PbTe) and their nanoscale heterostructures in order to overcome the challenges of harvesting these short-lived excitons for photovoltaics. We have synthesized the nanocrystals of PbSe and PbS using classical colloidal nanocrystal synthesis (Figure 4). Furthermore, one-dimensional nanowires, hyperbranched nanowires, nanowire heterostructures, and other complex nanowire structures possess some advantages in the collection of photogenerated carriers over isolated nanocrystals. While still confined in 2 dimensions, photocarriers can be collected along the axial direction of the wire. We have synthesized hyperbranched PbSe and PbSe nanowire network structures using a chemical vapor deposition (CVD) process and in situ generated lead VLS catalysts (Figure 4).

Figure 4. Nanocrystals and hyperbranched nanowires of PbSe and PbS for the investigation of multi-exciton generation in nanomaterials.

We also investigate nanoscale those silicide materials that are semiconducting, such as CrSi2, FeSi2, MnSi1.75, Mg2Si, for their enhanced thermoelectric energy conversion due to reduced dimensions. Unconventional synthetic pathways to bulk quantity nanostructured silicide and silicon materials are being developed to enable the practical applications of these nanomaterials in high performance thermoelectrics.